IV… Oxide, Cro2 S

APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 8 23 FEBRUARY 1998

Extrinsic giant magnetoresistance in chromium „IV… oxide, CrO2 S. Sundar Manoharana) Max-Planck Institut fur Mikrostrukturphysik, Halle (Saale), D-06120, Germany D. Elefant and G. Reiss Institute for Solid State and Materials Research Helmholtzstrasse 20, D-01069, Dresden, Germany J. B. Goodenough Center for Materials Science & Engineering, ETC 9, 102, University of Texas at Austin, Austin, Texas 78712-1063 ͑Received 22 October 1997; accepted for publication 9 December 1997͒

Polycrystalline CrO2 is shown to exhibit a giant magnetoresistance ͑GMR͒ at low temperatures. A rapid decrease in the GMR with increasing temperature is correlated with a decrease in the intergranular resistance. Single-crystal CrO2 is a half-metallic ferromagnet, as the data are interpreted to reﬂect two types of charge carriers, crossing a grain boundary: those that tunnel between conduction bands of adjacent grains and those that hop after residing at a localized state for a time long compared to a spin relaxation time. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03106-4͔

The electron carries in spin as well as a charge. Tradi- octahedral sites of a tetragonal rutile structure having the tionally, the electronics industry has exploited its ability to space group P42 /mmm of the three non-␴-bonding 3d or- control the transport of mobile electronic charges and the bitals per Cr atom symmetry which distinguishes a single orientation of stationary electron spins. Recently, interest in localized orbital directed along the c axis from the other two, the transport properties of electron spins has been stimulated which form a narrow, orbital quasidegenerate band as a re- by attempts to obtain a giant negative magnetoresistance sult of stronger Cr-O:2p-Cr interactions. With one electron ͑GMR͒ at room temperature in modest applied fields. Two in the localized orbital, a strong Hund’s intraatomic ex- mechanisms for achieving a GMR have been investigated: ͑i͒ change field removes the spin degeneracy of the bands; but Changing from antiferromagnetic to ferromagnetic in a mod- with one electron per Cr atom in these bands, the majority- est magnetic field where the coupling across a paramagnetic spin band remains only half filled, so ferromagnetic coupling layer P between alternate ferromagnetic layers in a remains compatible with occupancy of only bonding states. multilayer F/P/F stack supporting preferential spin transport The ferromagnetic coupling via the itinerant electrons domi- perpendicular to the layers. Control of the thickness of the P nates the antiferromagnetic superexchange interactions be- layers in a ‘‘magnetic superlattice’’ consisting of many al- tween next-near-neighbor Cr atoms along the c axis via the ternating layers has been needed to obtain an adequate sig- localized electrons. nal. Prototype magnetic readout heads for magnetic storage In this letter we report the observation of a GMR below 1 are already under active development. ͑ii͒ Changing the spin 77 K in polycrystalline CrO2. In a saturation applied field the transport across a F͉F interface by sweeping an applied field spins on either side of a grain boundary are oriented parallel, through the coercivity of the ferromagnetic layers; the inter- and the resistance is lowest; at the coercive field, misalign- facial layer which is parallel may be a grain boundary or a ment of spins on crossing a grain boundary is maximized, thin dielectric layer across which an electron spin tunnels and the resistance is highest. The origin of the sharp decrease with conservation of its spin angular momentum. This pro- in signal at higher temperatures is qualitatively modeled. cess has been demonstrated in the manganese-oxide perov- CrO2 powders were prepared hydrothermally as de- 4 skite La0.7Sr0.3MnO3, which is a half-metallic ferromagnet, scribed elsewhere. The particles were fine-grained needles but the signal is rapidly suppressed on raising the tempera- of acicular shape. Uniaxially cold-pressed samples of 10 mm ture above 77 K.2 In either approach, the signal would be diameter were prepared with thicknesses ranging from 1.5 to optimized where the ferromagnetic layers are half-metallic 3.0 mm; the magnetoresistance ratio varied little with pres- ferromagnets, i.e., where the itinerant electrons all have the sure from 300 to 400 MPa. Decomposition of CrO2 above same spin. In this case, parallel alignment of the half- 250 °C sets an upper limit to the sintering temperature. metallic ferromagnets allows transfer of electrons that retain Therefore, we have also prepared hot-pressed CrO2 samples their spin on crossing the P or interfacial layer whereas an- in a polymer matrix; the GMR of these samples will be pub- tiparallel alignment blocks these electrons. lished elsewhere.5 A scanning electron micrograph of the Chromium ͑IV͒ oxide, CrO2, is a room-temperature fer- cold-pressed samples indicates a porosity of less than 30%; 3 romagnet and, at Tϭ0 K, it is a half-metallic ferromagnet. the pellet had a density 70% of theoretical ͑4.899͒ g/cm3. 4ϩ 2 In an ionic model, CrO2 contains Cr :3d configurations in The electrical resistance was obtained with a standard four- probe technique; Ohmic contacts were made with Ag. The a͒For correspondence: Dept. of Chemistry, Indian Institute of Technology room-temperature specific resistivity of the cold-pressed Kanpur, 208 016, India. Electronic mail: [email protected] samples was 0.05 ⍀ cm, that of the hot-pressed samples was

0003-6951/98/72(8)/984/3/$15.00 984 © 1998 American Institute of Physics Downloaded 12 Feb 2001 to 195.37.184.165. Redistribution subject to AIP copyright, see http://ojps.aip.org/aplo/aplcpyrts.html. Appl. Phys. Lett., Vol. 72, No. 8, 23 February 1998 Sundar Manoharan et al. 985

FIG. 1. The temperature dependance of resistance of a cold-pressed CrO2 sample. Inset shows the dependence of the resistance at 4.2 K on magnetic ﬁeld applied parallel to the current direction.

10 ⍀ cm. The oxygen content, 38Ϯ0.05 wt %, of the cold- pressed samples was obtained by thermogravimetric analysis of the weight loss on oxidizing the sample to Cr2O3; it cor- responds to a molecular formula CrO2.01Ϯ0.02. Figure 1 shows the temperature dependance of the resistance R(T) normalized to the room-temperature resistance R(300 K) for a virgin ͑demagnetized͒ sample, 2.60 mm thick that was cold pressed at 400 MPa. Although the single- crystal samples are metallic,6 the cold-pressed samples have FIG. 2. The field dependence of the magnetoresistance ͑MR͒ ratio at differ- a negative slope of R vs T in the range 150ϽTϽ300 K that ent temperatures for ͑a͒ BϽ5 T and ͑b͒ BϽ1.1 T. The separation of the increases dramatically with decreasing temperature below peak resistances on the forward and backward sweep of B correspond to the 150 K. The hot-pressed samples, on the other hand, exhibit a coercivities ϮBc . positive slope of R vs T in the range of 150ϽTϽ300 K; the resistance then increases sharply with decreasing tempera- 2 La0.7Sr0.3MnO3 samples. In the latter case, it has been tures below 150 K. The negative slope of the R vs T curve of 7 Fig. 1 is clearly an extrinsic property associated with the shown to be related to the grain-boundary resistance rather intergranular resistance. than to a resistance associated with traversing a domain wall. The inset of Fig. 1 shows, for the cold-pressed sample, Given the dominant grain-boundary contribution to the resis- the dependence of the resistance at 4.2 K in a magnetic field tance of our CrO2 samples, the same conclusion would ap- B applied parallel to the current direction normalised to the pear to apply to the CrO2 samples also. resistance of the virgin sample in zero applied filed. Little Figure 2͑a͒ compares the magnetoresistance at 4.2 K difference was found if B was applied perpendicular to the with that at 50 K and 211 K. Saturation was not reached at current direction. On the initial magnetization to BϭϪ5T Ϯ5 T at any temperature; CrO2 is a hard ferromagnet. At ͑straight line in inset͒, the resistance R(B) at 4.2 K was room temperature, there is little change in the resistance with higher than the R(B) curves taken on subsequent cycling of magnetic field for BϽ5 T. This observation is consistent 6 B between Ϫ5 T. This behavior reflects the removal of mis- with an earlier report by Rodbell et al. that the drop in oriented magnetic domains and the reorientation of the room-temperature resistance of a hot-pressed CrO2 sample atomic spins in the magnetic field; the magnetic domain con- was less than 1.5% in an applied field Bϭ2.7 T. However, figuration after the initial magnetization is different from that below 150 K the magnetoresistance increases significantly of the virgin sample, and the 180° domain walls that are with decreasing temperature down to 4.2 K the lowest tem- nucleated and/or move in the sample with changing B are perature at which measurements were made. The magnetore- different after magnetization than in the virgin samples. The sistance ratio ͓R(5 T)ϭR(Bϭ0)͔/R(5 T) increases sharply sharp maxima in the R(B) /R(Bϭ0) curves in subsequent with decreasing temperature see Fig. 3, from 3% at 210 K to cycles of B between Ϯ5 T are found at the coercive field 42% at 4.2 K. Figure 2͑b͒ shows similar curves for B ϮBc , where the domain wall density is a maximum. These Ͻ1.1 T taken at 4.2, 10, 50, and 211 K. The coercivity ob- data clearly demonstrate that the resistance increases with the tained from the separation of the peak resistances on the density of misaligned domains facing each other across a forward and backward sweep of B is BcϭϮ0.11 T for a B grain boundary. Such a thermal hysteresis effect is also char- Ͻ1.1 T. On lowering the maximum applied field to B 5 acteristic of the hot-pressed CrO2 samples and of thin-film Ͻ0.02 T ͑200 g͒, an appreciable magnetoresistance was re- Downloaded 12 Feb 2001 to 195.37.184.165. Redistribution subject to AIP copyright, see http://ojps.aip.org/aplo/aplcpyrts.html. 986 Appl. Phys. Lett., Vol. 72, No. 8, 23 February 1998 Sundar Manoharan et al.

ers crossing a grain boundary. At higher temperatures, the multistep hopping electrons dominate the conductivity and the GMR effect of the single-step electrons becomes over- whelmed by the conduction of electrons that lose their initial spin orientation. In summary a giant negative magnetoresistance ͑GMR͒ has been found below 77 K in cold-pressed, polycrystalline CrO2 samples. It does not occur at the Curie temperature (Tcϭ125 °C) as does the intrinsic colossal magnetoresistance ͑CMR͒ found in certain manganese oxides with the perovskite structure. It is to be compared with the extrinsic GMR found at low temperatures in La0.7Sr0.3MnO3. In prin- ciple, the extrinsic GMR should be optimized at the coercive field B of a polycrystalline half-metallic ferromagnet with a relative square B-H hysteresis loop. However, realization of the effect requires that the conduction electrons tunnel in a single step from the conduction band of one grain to that of FIG. 3. Temperature dependence of the MR ratio for the cold-pressed CrO2 the other across the grain boundary. If the electrons are sample; solid line is a guide to the eye. trapped for a time long compared to the spin relaxation time ͑either at surface states or at intermediate trapping centers͒ tained; 20%, 11%, 5% at 4.2, 10, and 50 K, respectively. No during traversal of a grain boundary, they lose their spin appreciable difference in the magnetoresistance was found information and cannot contribute to the GMR effect. At on varying the sample thickness from 1.5 to 3.0 mm. lowest temperatures, filling the trap states with immobilized Comparison of Fig. 1 and Fig. 3 shows that the magne- charge carriers makes dominant the carriers that tunnel from toresistance ratio increases with the resistance in the virgin conduction band to conduction band across a grain boundary sample, which indicates that the intergranular resistance de- in one step; these electrons must dominate at higher tempera- pends sensitively not only on the alignment of the magneti- tures if a grain-boundary GMR mechanism is to be realized zation on either side of a grain boundary, but also on the at room temperature. concentration of charge carriers traversing a grain boundary. The dramatic increase in intergranular resistance with de- One of the authors ͑S.S.M.͒ thanks the Max-Planck Ge- creasing temperature suggests a simple qualitative model in sellschaft for a visiting fellowship, and Professor Kirschner which two types of electrons traverse a grain boundary: those for his encouragement and interest in this work. Initiation that hop in a succession of steps and a smaller number that grant from IIT Kanpur is greatly acknowledged. One of the tunnel across in one step between the conduction bands of authors ͑J.B.G.͒ thanks NSF for the financial support. the two grains. The latter would continue to conduct at lowest temperatures and would conserve their spin angular mo- 1 A. Hellemano, Nature ͑London͒ 273, 881 ͑1996͒. mentum in a single jump; these are the primary contributions 2 J. Z. Sun, W. J. Gallagher, P. R. Duncombe, L. Krusin Elbaum, R. A. to the GMR. The electrons that traverse a grain boundary in Altman, A. Gupta, Yu Lu, G. Q. Gong, and Gang Xiao, Appl. Phys. Lett. two or more steps have time to relax their spin orientation 69, 3266 ͑1996͒. 3 between jumps, in which case they make no contribution to J. B. Goodenough, Prog. Solid State Chem. 5, 141 ͑1972͒. 4 T. S. Kannan and A. Jaleel, Indian Patent No. 627/Del/87. the GMR. At low temperatures, the electrons that move in 5 S. Sundar Manoharan, A. Daniel, D. Elefant, and G. Reiss ͑submitted for successive steps become increasingly trapped out on lower- Patent͒. ing the temperature, and the single-step electrons contribute 6 D. S. Rodbell, J. Phys. Soc. Jpn. 21, 1224 ͑1966͒, D. S. Rodbell, J. M. an increasing fraction to the conductivity. Therefore, the Lommel, and R. C. De Vries, ibid. 21, 2430 ͑1966͒. 7 N. D. Mathur, G. Burnell, S. P. Isac, T. J. Jackson, B. S. Teo, J. L. Mac GMR increases at low temperatures where the electrons that Manus-Driscoll, L. F. Cohen, J. E. Evetts, and M. G. Blamire, Nature tunnel in a single step are the dominant mobile charge carri- ͑London͒ 387, 266 ͑1997͒.

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